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Newly published in the MSDN library, this documentation and accompanying sample application will get you started building a complete application that targets Windows Phone OS 7.0. You will learn about common developer issues in the context of a simple fuel-tracking application for your car named Fuel Tracker. This topic describes things you should know before you start creating your Windows Phone application and includes a list of other useful resources to help you out.
Among the topics:
Get started here.